SGP reporting different sky angle than with prior version of ASTAP

[Patrick Spencer]

Have there been any recent changes to the way ASTAP calculates and/or reports the position angle of solved images? I use ASTAP as my platesolver in Sequence Generator Pro. I recently updated ASTAP from 2022.08.22 to 2022.09.17. When using the new version, after a running a solve and sync command, the sky angle displayed in SGP is now different from the the sky angle displayed when using the old version.

I attached two screenshots to illustrate the issue. In both cases, a solve and sync operation was performed with the same mechanical rotation angle and pointing at the same spot in the sky. The only change was I uninstalled 2022.09.17 and reinstalled v 2022.08.22.

Thanks for any insight you can provide,

Patrick Spencer

[han.k]

Hi Patrick,

Yes there angle calculation has been revamped. There was a problem with the angle for flipped images. Is there any problem you can report?

Han

[Patrick Spencer]

Hi Han,

Thanks for the quick reply. This did cause me to have a problem. I had reset the desired rotation angle of a target in the sequence to match the "new" sky angles that SGP was showing. When that target started, SGP could not rotate and solve properly, so it flailed around for for a while before finally giving up and aborting the sequence. That occurred at 1:30 AM, so I was asleep and not available to manually rescue the run.

Patrick

[han.k]

Hello Patrick,

I'm sorry for the nightly problems.

I have reviewed this morning the angle reporting and decided to document is better. The old fashioned style solution contains CROTA2, CROTA1, CDELT2, CDELT1 which is rarely reported. Normally CROTA2 is reported as then angle but you can flip the angle by reporting CDELT2 the pixel size negative. Attached a screenshot of which I will add to the documentation . The last column indicates the old ASTAP CROTA2 reporting after solving.

It is easy to make a mistake in the math. Please report any other problem.

Han

[han.k]

Here the new description in the documentation:

http://www.hnsky.org/astap.htm#viewer_angle

[Patrick Spencer]

Hi Han,

Thank you so much for providing this information. I really appreciate your help. I will update my targets in SGP to reflect the new convention regarding position angles.

By chance, do you have a formula I could use to make a "crosswalk" between the old values and the new values? A spreadsheet would help make sure I don't make any mistakes when translating the numbers.

Thanks again,

Patrick

[Patrick Spencer]

Hi Han,

In case it wasn't clear, I wasn't asking you to make a spreadsheet, I just wondered about the formula so I can make one...

;-)

[han.k]

These exercises are always good to double check the math.

Tracing back the math is not so easy but looking to the table I posted earlier, see red values, I think you should do the following:

0<old Angle<90 ==> add or subtract 180 degrees.
270<old Angle<360 ==> add or subtract 180 degrees.

So for the image it doesn't matter, 180 degrees will only rotate the image but will not effect the stacking. It is like a meridian flip. So it is for me unclear why SGP has problem with it.

Han